1. Field of the Invention
The present invention is relative to single phase AC to DC converter with power factor correction function (so that the power factor of the input current is at a power factor in excess of that of an otherwise comparable low-power-factor converter design). It can be used on in switching mode power supply and electronics ballast.
2. Description of the Prior Art
The demand for and development of power factor correction (PFC) circuit has been fueled by a concern over the massive use of electronics power converter, such as, AC-DC-DC, AC-DC-AC employed in switching mode power supply system. Due to cost and efficiency consideration, it is desirable to employ a simple PFC circuit and increase the efficiency of the whole system.
PFC circuits are classified into two groups. First group is defined as active PFC circuits, and second group is defined as passive PFC circuits. The very popular Boost-type PFC circuit is an active PFC circuit. It can shape the input current and make the total harmonic distortion (THD) very low. However, the efficiency of the active PFC circuit is lower than one of passive PFC circuit, due to extra switching circuit. Further, the control of the active PFC is complicated, resulting in increased manufacturing cost and reduced reliability of the circuit. For passive PFC circuit, due to no active control switch in the circuit, the passive PFC can work in higher efficiency, but THD of the passive PFC is higher and size of the passive components is big.
Based on the advantage and disadvantage of two groups PFC circuits, the concept of single power stage converter with PFC was presented for several years. In the converter, some extra passive components are added to a regular converter. The extra components are working in the converter's switching frequency. The size of the extra component is small due to higher operating switching frequency. In this kind of converter, the main task of the active switch of the converter is to regulate the output power. The active switch involves a part of task to shape the input current. Due to both input and output current controlled by the active switch, the loss on the active switch is higher and the efficiency of the whole system is lower.
Based on the existed PFC circuits, there are a lot of papers and patents about valley-fill circuit. The basic valley-fill circuit is shown in FIG. 1. Valley-fill circuit can provide better performance than other passive types of PFC circuits.
In the valley-fill circuit, the power line directly feeds energy (e.g. electrical energy) to the load through the rectifier diodes for approximately 120 degrees around the peek voltage. Two storage capacitors C1 and C2 feed energy to the load through diodes D1 and D2 for approximately 60 degrees near the zero line crossing points. Most of the input energy being first fed to the load, with a small portion of the input energy being first fed to the two storage capacitors C1 and C2, and then fed to the load through capacitors C1 and C2. As a result, such a circuit offers a relatively high operating efficiency.
Problems with the valley-fill circuit are a pulsating line current charges the capacitors near the peak power line voltage, resulting in a deteriorated PF (of about 0.95) and a high THD (e.g. about 40%). The output of the valley-fill circuit exhibits a large ripple from the half of the power line peak voltage to the power line peak voltage, with the ripple frequency being equal to twice the line frequency.
A great deal of the time and effort has been spent in attempts to improve the PFC performance of the valley-fill circuit. This work has been directed to shaping the input current during the approximate 60 degree dead time near the zero line crossing points, and to limiting the pulsating line current that charges the capacitors near the peak line voltage.
A paper titled “A Unity Power Factor Electronic Ballast for Fluorescent Lamp Having Improved Valley Fill and Valley Boost Converter” from Conference Record PESC'97, describes the use of an active boost circuit to shape the input current during the approximate 60 degree dead time near the zero line crossing points, as shown in FIG. 3. Because a boost switch still suffers the peak input voltage and the switch only works during the 60 degree dead time, as shown in FIG. 4, a complex control method is required to detect the operating point. In addition, the complexity of the circuit decreases the reliability and increases the total manufacturing cost.
Japanese Pat. No. HEI 8-205520 illustrated in FIG. 5, describes the load current of a PFC converter as being discontinues, and discloses that the insertion of a suitable inductor L1 in the input power line avoids pulsating of the power line current. Because an instantaneous line voltage is higher than the voltage of each DC bulk capacitor C1 and C2, while being less than the sum of the voltages of the two capacitors, the inserted inductor provides a boost function to boost the sum of the voltage of the two capacitors. However, this disclosure fails to solve the above-described problem that exists at the input current during the approximate 60 degree dead time near the zero line crossing points.
U.S. Pat. No. 5,986,901 illustrated in FIG. 6, discloses the use of the high frequency discontinues input current of the converter to drive a charge pump circuit Z and the inserted input inductor. As shown in FIG. 6, the charge pump circuit shapes the input current during the approximate 60 degree dead time, and the input inductor provides a boost function to boost the sum of the voltage of the two capacitors. Because the charge pump circuit and the inserted input inductor are driven with the discontinues input current automatically, the active switch or switches in the converter would not be exposed to extra current or voltage stresses. However, the disclosure needs several passive components to implement the charge pump circuit. The cost of the disclosure is still high.